High Voltage Testing and Engineering Commission

Fachkommission für Hochspannungsfragen

Cable testing

Proof of proper assembly and installation of the cable systems is the primary consideration

when carrying out on-site voltage testing of high-voltage cable connections.

The on-site AC voltage testing technology for high-voltage cables that is now used throughout

the world was developed jointly by the Swiss Federal Institute of Technology Zurich (ETH) and

FKH, and was introduced by FKH in the 1980s. It is based on the series resonance principle

with a tuneable frequency. Various other diagnostic methods, including partial discharge (PD)

measurements in particular, are offered in addition to voltage testing.

High-voltage series-resonant testing of cable systems

Prior to commissioning a new cable system, or after the completion of major conversion work

on an existing system, voltage testing is recommended before starting to operate the system.

The aim is to exclude the following faults in particular:

Assembly errors in cable terminations or joints

Damage to the cable sheath or the semiconding screen

In compliance with CENELEC HD 632 and in divergence from IEC 60840 and/or 62067, FKH

recommends that a higher test voltage should be applied to cables, but for a reduced testing

duration. This recommendation is based on FKH's lengthy experience of testing.

Recommended test procedure depending on the system voltage

In the test circuit, an inductance L with a high quality factor is connected in series to the test ca-

pacitance Cx of the object. Under resonance conditions, a voltage UP builds up over the test ob-

ject that is considerably greater than the voltage UT of the supply transformer. The test frequency

is obtained with the following formula:

Thanks to its modular inductances, the test system can be configured for a sufficiently high test

frequency to ensure that voltage transformers do not reach magnetic saturation; this eliminates

the need to remove them for the test.

Diagram of a series-resonant system

U / U0 Utest Utest/ U0 Test

duration

45 / 26 kV 57 kV 2.2 15 min

60 / 36 kV 80 kV 2.2 15 min

110 / 64 kV 140 kV 2.2 15 min

132 / 76 kV 167 kV 2.2 15 min

150 / 87 kV 190 kV 2.2 15 min

220 / 127 kV 254 kV 2.0 15 min

275 / 160 kV 320 kV 2.0 15 min

380 / 220 kV 400 kV 1.8 15 min

totCLf

=

2

1

Series-resonant systems are virtually the only possible high-voltage sources for AC voltage tests

on lengthy XLPE- and EPR-insulated cable sections with typical capacitance values (conductor /

to screen) of 0.1 to 0.5 µF per km. This restriction is due to the high charging current IC and/or the

high reactive power Q. Oil cable systems can also be tested according to this principle.

Advantages over other methods

Power supply required is low

Voltage at the test object is sinusoidal and free of

harmonics

Minimal energy release at the fault location in case of

test object breakdown

If the inductances are connected in parallel, the

frequency can be increased so that voltage transform-

ers can be included in the testing

Additional cable connections and other equipment

connected via switchgear can also be included in the

testing

Concept for integral testing of complete systems

Partial discharge measurement and other diagnostic methods

A partial discharge measurement is recommended as an additional measure for important cable sections and higher voltage levels. This involves specific inspections of the cable termi-nations and joints in order to identify insulation faults. In this way, it is possible to detect flaws that have not yet caused an immediate breakdown, but which harbour the risk of an unex-pected insulation failure.

FKH uses the following partial discharge test methods and coupling types: :

Conventional, via a coupling capacitor

With partial discharge sensors integrated into the fittings (joints, terminations)

Via current sensors on the cable shielding

With acoustic emission sensors

For oil-insulated cables and in the medium-

voltage grid, FKH also performs C-tgδ mea-

surements as well as measurements of the

polarisation and depolarisation currents

(PDC).

The modern partial discharge measure-

ment instruments used by FKH allow syn-

chronous measurements at different points.

To capture all partial discharges simultane-

ously, fibre-optic (OWG) connections must

be installed between all the sensors in

advance.

High-voltage resonance test on a 275 kV

cable connection

Partial discharge level during a voltage test

Cable testing system in outdoor switchgear

Series resonance system on an offshore

wind power converter platform with test ca-

ble adapters

Complete experimental setup for cable testing

with partial discharge measurement in the

gallery of a hydroelectric power plant

FKH Head Office

Hagenholzstrasse 81

8050 Zurich

FKH Insulating Oil Laboratory

4658 Däniken

FKH Experimental Station

4658 Däniken

Phone: +41 62 288 77 99

Fax: +41 62 288 77 90

Phone: +41 44 253 62 62

Fax: +41 44 253 62 60

Phone: +41 62 288 77 95

Fax: +41 62 288 77 94

www.fkh.ch / info@fkh.ch

HIGH VOLTAGE TESTING AND ENGINEERING COMMISSION

@ FKH-2021-01